Printing and printers

ABSTRACT

A method of printing a document using one of a plurality of print modes, the method comprising:
         (i) notionally dividing a digital image of the document into sectors;   (ii) for each sector, comparing one or more properties of the luminosity of pixels within each sector with one or more threshold values;   (iii) using the comparison of (ii) to determine whether each sector is likely to show an image defect if printed at a lower image quality print mode;   (iv) selecting a print mode such that all sectors are determined not to be likely to show an image defect;   (v) printing the document using the determined print mode.

RELATED APPLICATIONS

This patent application claims priority to applicationPCT/EP2005/055429, having title “PRINTING AND PRINTERS”, filed on 20Oct. 2005, commonly assigned herewith, and hereby incorporated byreference.

FIELD OF THE INVENTION

This invention relates to printing and printers.

BACKGROUND

Digital printers, such as inkjet, laser printers, dot matrix printers,LED printers, or gel printers, (to name a few) have been around for manyyears. They can be black and white or colour printers (e.g. four colour,or six colour, or more). They print onto a print medium, typicallypaper, cardboard, plastics, film or the like. Paper comes in differentkinds with different colours and different ink-retention properties.

Inkjet printing is a non-impact printing process in which droplets ofink are deposited on a print medium in a particular order to formalphanumeric characters, area-fills, and other patterns. Low cost andhigh quality of the hardcopy output, combined with relatively noise-freeoperation, has made inkjet printers a popular alternative to other typesof printers used with computers.

Inkjet printing involves the ejection of fine droplets of ink onto aprint medium such as paper, transparency film, or textiles in responseto electrical signals generated by a microprocessor. There are two basicmeans currently available for achieving ink droplet ejection in inkjetprinting: thermally and piezoelectrically. In piezoelectric inkjetprinting, the ink droplets are ejected due to the vibrations ofpiezoelectric crystals, again in response to electrical signalsgenerated by the microprocessor.

In thermal inkjet printing, an inkjet image is formed when a precisepattern of dots is ejected from a drop generating device known as a“print head” onto a printing medium. The typical inkjet print head hasan array of precisely formed nozzles (or ejector portions) attached to athermal inkjet print head substrate, such as silicon, nickel, orpolyamide, or a combination thereof. The substrate incorporates an arrayof firing chambers or drop ejector portions that receive liquid ink(colorants dissolved or dispersed in a solvent) through fluidcommunication with one or more ink reservoirs. Each firing has a filmresistor, known as a “firing resistor” located opposite the nozzle soink can collect between the firing resistor and the nozzle. The printhead is mounted on a carriage that travels along the width of theprinter (otherwise referred to as the “scan axis”).

Digital printers have a number of “print modes” configured in them atthe source of manufacture to set parameters and variables to ensure thata satisfactory print job is achieved. This can include ensuring that thecorrect ink load is applied when printing on a specific media, and thata required Image Quality (IQ) is produced. Other parameters that aredefined by the print mode can include the number of passes of a printerhead over a print medium, the dots per inch (dpi) that defines thenumber of dots of ink that are printed onto a page, or values in aLook-Up-Table (LUT) that define properties of ink composition for theprinted page, for example how different coloured inks are mixed togetherto produce a desired colour.

Further parameters include the carriage speed and firing frequency ofthe print head. The carriage speed is measured in inches per second(ips), at which the printer carriage (and the print heads in it) moveacross the media. Typical carriage speeds range from 20 to 60 ips. Thefiring frequency is the number of drops fired per second. The higher thefiring frequency, the higher the dot placement error of the drops on themedia. High dot placement errors provoke undesired IQ defects (grain inarea fills, blurred lines, etc.). Firing frequency is a parameter thatis closely linked to the number of passes of the print mode (the higherthe number of passes, the lower the firing frequency) and to thecarriage speed (the higher the carriage speed the higher firingfrequency of the pen).

The number of passes of the printer head over a print medium defines thenumber of times that the printer head passes over the same region of thedocument whilst applying ink to the medium. Each time a subsequent passof the printer head is made, ink is applied on top of ink that has beenapplied in an earlier pass of the printer head.

Each print mode will have a different combination of values for theparameters that control aspects of the print operation. For example, oneset of print modes may have constant values for all parameters exceptthe number of passes, where each print mode defines a different numberof passes of the print head that are performed during a print operation.There may also be other sets of print modes where the number of passesis kept constant and another parameter, for example dpi, is set atdifferent values in each of the print modes within the set. Having anumber of different print modes enables a user to select a print modethat strikes a balance between throughput of print jobs and imagequality of the print job.

The wrong selection of the print mode can be two-fold. On one hand theuser may select higher image quality print modes than needed. Thiscauses a waste of time due to the slow throughput of print jobsassociated with these print modes. This is because the printer takes alonger time to perform a print operation with a higher image qualityprint mode than a lower image quality print mode, as more ink isapplied, and/or more passes of the printer head are typically performed,when performing a print operation with a higher image quality printmode.

On the other hand, if the user selects a lower image quality print modethan required, it is likely that the final printout will not meet theimage quality expectations, especially due to a defect known as“banding” as will be discussed in more detail later.

In some cases a printed medium may have to be scrapped (thrown away) ifit does not have a high enough image quality and the user will be forcedto repeat the print job with an associated waste of money and time.Selection of the correct print mode helps to achieve optimal, or better,performance.

Some digital printers accommodate different kinds of paper/media byhaving a plurality of different print modes, different print modes beingassociated with specific types of media. Some printers can also haveprint modes associated with more than one print quality for eachsupported media, for example greyscale, normal and photo quality. Forexample, if there are three types of media supported and three printqualities available, the printer will have nine different print modes.

The user can select an appropriate print mode to be used for printing aparticular print job with a particular printer from a group or set ofpre-set possibilities in accordance with which media will be used andwhich print quality is required.

Typically users lack the knowledge and understanding to correctly selectthe optimal print mode for their particular needs. Moreover, the mediaportfolio can be very large and often confusing. Sometimes users choosenot to spend the time necessary to select the correct print mode. Thisis a recognised usability issue.

A user may select the print mode that provides the best possible imagequality in order to avoid the risk that an unacceptable image qualitywill be produced. Selecting an unnecessarily high print mode isinefficient and reduces the throughput of print jobs that could beachieved if the correct print mode were selected. Some print jobs can beprinted faster, still with acceptable results, with a lower imagequality print mode. Selecting an unnecessarily high image quality printmode further wastes ink and printing time, as ink is printed onto themedium that does not substantially improve the image quality, orimproves the image quality to an unnecessarily high level.

Alternatively, a user may trial and error with different print modes todetermine the most efficient print mode that provides them with anacceptable image quality. This also wastes time and ink during thetrials, and also paper with associated cost and refuse disposal issues.

An unacceptable image quality can include an effect known as “banding”.Banding occurs where an insufficient amount of ink has been printed ontothe medium, and lighter colour bands are created across the image wherethere is a shortage of ink. Banding shows up as straight bands or linesthat follow the direction of the printer carriage motion. Alternatively,banding may be caused where ink is printed onto incorrect regions of themedium, and some areas do not have the ink that they should have.

There are a number of causes of banding, and the most common cause ofbanding is where an incorrect print mode has been selected. Theincorrect print mode causes too little ink to be printed onto themedium, or causes the ink to be printed onto the medium in too fewpasses of the print head over the medium (for example, in an inkjetprinter, such as a thermal or piezoelectric inkjet printer).

It is easy to see banding in low pass print modes. High pass print modescan be very robust to this type of image quality defect.

Banding can also occur where a nozzle in a print head is blocked, wherea nozzle is not aligned correctly and therefore does not fire the inkonto the correct region of the media, or the nozzle is otherwise notfiring correctly. Whether or not banding occurs is also influenced bythe accuracy/quality of the print medium, as this determines how muchink is required for the print medium to provide an acceptable imagequality. For example, the porosity of the print medium, matte or glossyfinish, or colour of the print medium can alter the desired print mode.

Although the invention is particularly concerned with inkjet printing,the invention may also find application with regard to other printingtechnologies, such as laser printing, LCD printing, LED printing, LEPgel printing, and/or toner-based printing (e.g. photocopiers), and otherprinters. The list above is not exhaustive.

According to a first aspect of the invention, there is provided a methodof printing a document (100; 400) using one of a plurality of printmodes, the method comprising:

-   -   (i) notionally dividing a digital image of the document (100;        400) into sectors (102, 202, 302; 402, 502);    -   (ii) for each sector (102, 202, 302; 402, 502), comparing one or        more properties of the luminosity of pixels within each sector        with one or more threshold values;    -   (iii) using the comparison of (ii) to determine whether each        sector (102, 202, 302; 402, 502) is likely to show an image        defect if printed at a lower image quality print mode;    -   (iv) selecting a print mode such that all sectors (102, 202,        302; 402, 502) are determined not to be likely to show an image        defect;    -   (v) printing the document (100; 400) using the selected print        mode.

Comparing a property of the luminosity of pixels within each sector mayinvolve determining the mean value and/or standard deviation of theluminosity of the pixels and comparing it/them with one or morethreshold values.

Thus a document can now be printed with an optimal print mode that hasbeen determined dynamically and automatically, without a user's input,using information obtained from the document that is to be printed. Thiscan avoid a user accidentally (or deliberately) selecting aninappropriate print mode for printing the document. Also, it is notnecessary for a user to select any print mode: the printer/processorautomatically selects a suitable print mode.

Another aspect of the invention relates to a printed document producedusing the method of the first aspect of the invention.

According to another aspect of the invention, there is provided aprinter system comprising a printer, computer memory having a pluralityof print modes stored thereon, and printer driver software;

-   -   the system being configured such that the printer driver        software is adapted to compare one or more properties of the        luminosity of pixels within a document that is to be printed        with one or more threshold values, and to select one of the        plurality of print modes that is to be used to print the        document based on the comparison.

According to a further aspect of the invention, there is providedprinter control software which is adapted, when loaded on a processorassociated with a printer, to compare at least one property of theluminosity of pixels in a document that is to be printed with at leastone threshold value, and to select one of a plurality of print modesthat is to be used when printing the document based on the comparison.

The software may be stored on a processor that is located on theprinter, or may be located in an off-printer processor that is incommunication with the printer.

According to another aspect of the invention, there is provided aprinter system comprising means for printing, and means for dataprocessing, the system being such that, in use, the means for processingcompares a property of the luminosity of pixels within a document thatis to be printed with a threshold value and determines an optimal printmode associated with the document, and the means for printing prints thedocument using the determined print mode.

According to a further aspect of the invention there is provided amethod of calibrating a printer comprising analysing properties of adocument that is to be printed, and calibrating printer settings basedon the analysis.

According to a further aspect of the invention there is provided amethod of printing a document using one of a plurality of print modes,the method comprising:

-   -   analysing a property of the luminosity of pixels within the        document;    -   selecting an optimal print mode for printing the document from        one of the plurality of print modes based on the analysis;    -   printing the document using the determined print mode.

According to a further aspect of the invention there is provided amethod of reducing the likelihood of image quality defects in a printeddocument comprising:

-   -   notionally dividing the document into a grid of sectors;    -   comparing at least one property of the luminosity of pixels        within each sector with at least one threshold value;    -   associating an individual optimal print mode associated with        each sector based on the comparison for that sector;    -   determining an optimal print mode for the document as a whole        based on the individual optimal print modes associated with each        sector; and    -   printing the document using the optimal print mode; wherein the        optimal print mode provides a print job with reduced image        quality defects when compared with a print job provided by other        print modes.

According to a further aspect of the invention there is provided amethod of selecting a print mode for printing a document comprising:

-   -   analysing at least one property of the luminosity of pixels        within the document;    -   comparing the analysis of the at least one property of the        luminosity of the pixels with at least one threshold value; and    -   selecting a print mode for printing the document based on the        result of the comparison.

According to a further embodiment of the invention, there is provided amethod of printing a document (100; 400) using one of a plurality ofprint modes, the method comprising:

-   -   notionally dividing the document (100; 400) into sectors (102,        202, 302; 402, 502);    -   comparing one or more physical characteristics of pixels within        each sector as they will appear in the printed document, with        one or more threshold values;    -   determining one of the plurality of print modes as being the        minimum print quality suitable for printing that sector based on        the comparison of the physical characteristics of pixels for        that sector;    -   determining the optimal print mode for the document based on the        minimum print modes determined to be suitable for each sector;        and    -   printing the document using the determined optimal print mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Various examples of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 shows a schematic representation of a printer according to anembodiment of the invention;

FIG. 2 shows a schematic representation of a system for printing adocument according to an embodiment of the invention;

FIG. 3 shows a flow diagram of the steps carried out when printing animage according to an embodiment of the present invention;

FIG. 4 illustrates a stage in the processing of an image to be printedaccording to an embodiment of the present invention;

FIG. 5 illustrates a further stage in the processing of the image ofFIG. 4 according to an embodiment of the present invention;

FIG. 6 illustrates a further stage in the processing of the image ofFIG. 4 according to an embodiment of the present invention;

FIG. 7 illustrates a further stage in the processing of the image ofFIG. 4 according to an embodiment of the present invention;

FIG. 8 illustrates a stage in the processing of another image to beprinted according to an embodiment of the present invention; and

FIG. 9 illustrates a further stage in the processing of the image ofFIG. 8 according to an embodiment of the present invention.

DETAILED DESCRIPTION OF SOME EXAMPLES OF THE INVENTION

In use a printer is adapted to print indicia (e.g. words/text,pictures/drawings, photographs/representations of photographs, artwork,borders, lines, shapes or decorative features) onto a print medium, suchas one or more sheets of paper. The term print medium used hereingenerally includes media such as card, plastic or other material sheetor film, transparencies, photographic paper, labels, iron-on transfermaterial, and matte paper/glossy paper, as a non-exhaustive list ofexamples.

It is to be appreciated that by the term ‘printer’ we intend to coverequipment that is capable of printing ink onto a print medium andaccordingly includes equipment such as multiple-function machines (e.g.a combined printer, scanner and photocopier), fax machines and otheropen-media systems.

The term “ink” is meant to include liquid inks, gel inks and powder inks(e.g. toner that needs heat to fuse to a page/surface) and gels: it isnot used in a sense to restrict its physical form. Ink marks an articleto be printed and its application can be controlled by the printer.

A print operation can be considered as the physical act of applying inkto a medium in order to print indicia. A print job can be considered asthe physical medium on which the indicia has been printed. The print jobis produced by a print operation. In some embodiments, the printoperation may also comprise a processor (on or off printer) sendingelectronic signals to a print head to cause the indicia to be printed.Furthermore, the print operation may include an off-printer processorreceiving instructions from a user to print indicia, and sending anelectronic signal to the printer to cause the indicia to be printed.

A number of print modes are available to the printer that control,amongst other things, the number of passes that the print head makesover the medium, and the amount of ink that is fired onto the medium ineach pass of the print head.

FIG. 1 shows an inkjet printer 4 according to an embodiment of thepresent invention comprising a printer head assembly shown generally at10, a central processing unit 11, a memory 12 (which preferablycomprises both re-writeable memory (e.g. RAM, EEPROM) andnon-re-writeable memory (e.g. ROM)), at least one (or more)communications port 14, for example enabling connection to a PC, laptop,PDA or a network, and/or being adapted to receive a portable machinereadable data carrier such as a memory card (e.g. Memory Stick™) USBstorage devices, CDs, DVDs, flash memory cards. The printer 4 furthercomprises a paper tray 13 which stores a supply of paper to be printedon, and a paper feed assembly (not illustrated) to convey the paper fromthe paper tray 13 to the printer head assembly 10. Printer driversoftware 11 a is stored on the central processing unit 11.

In the memory 12 of the printer 4 there are stored two print modes, ahigh image quality print mode that causes the print head 10 to perform 8to 12 passes, in this embodiment 10 passes, over the print medium whenperforming a print operation and a low image quality print mode thatcauses the print head 10 to perform 4 to 6 passes, in this embodiment 5passes, over the print medium when performing a print operation. Theseprint modes cause a set amount of ink to be fired each time the printhead 10 passes over the print medium. The print head 10 prints a certainamount of ink onto the paper during each pass over the print medium,therefore, the more passes that the printer head makes over the printmedium, the more ink that is printed onto the print medium.

In some embodiments, a high image quality print mode can be consideredas a slower, “banding-free”, print mode, and a low image quality printmode can be considered as a “faster” print mode.

In other embodiments, there may be any number of print modes that definevarious qualities of print jobs. For example, there may be 3, 4, 5, ormore print modes that define varying qualities of print job.

The print modes may define any number of passes of the printer head overthe print medium, from 1 for a low image quality print mode up to 10,12, 15, or more, or any number in-between, for a high image qualityprint job.

The print modes may also define varying amounts of ink that are firedduring a pass of the print head. A low image quality print mode maycause a small amount of ink to be fired during a pass of the print headand a high image quality print mode may cause a large amount of ink tobe fired during a pass of the print head.

In other embodiments, the print modes may be stored in an off-printermemory and may be accessible by the printer 4 via the communicationsport 14.

A low image quality print mode may consist of 4 to 6 passes of the printhead over the print medium. A high image quality print mode may consistof 8 to 12 passes of the print head over the print medium. A low imagequality print mode may be faster than a high quality image print mode,and also use less ink than a high image quality print mode. It will beappreciated that interim print modes, having interim numbers of passesof the print head over the print medium, may be selected for images thatare determined to require an interim print mode.

In some embodiments a low image quality print mode, having say 6 passesof the print head over the print medium, may be capable of firing thesame amount of ink as a high image quality print mode, having say 10passes of the print head over the print medium. However, if analysis ofthe document that is to be printed determines that a high image qualityprint mode having 10 passes is required, but a low image quality printmode is used that has 6 passes, but fires the same amount of ink as thehigh image quality print mode having 10 passes, other defects in theprinted image may occur. For example, some images may be susceptible tograin if too few passes of the print head over the print medium aremade; grain occurs where colour within the image becomes grainy and doesnot look smooth as intended.

In some embodiments, a pen within the printer head can have a maximumamount of ink that it can fire in a single pass over the print medium,and if a print mode that has too few passes over the print medium isselected, it will not be possible to fire sufficient ink onto the printmedium without image quality defects occurring.

When a user initiates a print job (e.g. by clicking on “print” on thedisplay of their printer-connected computer) an instruction to print animage, and the electronic data representative of the image, is receivedvia the communications port 14. The electronic data representative ofthe image is then stored in memory 12. Alternatively, electronic datarepresentative of an image may already be stored in printer memory 12,and a user can select the image to be printed through a user interface(not shown) on the printer itself. It will be appreciated that an imageis not limited to a graphical image or photograph, and may consist ofany document that can be printed, including text, drawings, graphs,charts etc. or any combination thereof.

The printer driver software 11 a is adapted to analyse the image that isto be printed, and to determine from that analysis an optimum print modefrom the two print modes stored in memory 12 for printing the image. Ifthe printer driver software 11 a determines from the analysis thatbanding (or any other defect in image quality) is unlikely for theimage, then the low image quality print mode is selected from memory 12.If the printer driver software 11 a determines that banding (or anyother defect in image quality) is likely for the image, then the highimage quality print mode is selected from memory 12. A more detaileddiscussion of the analysis of an image is included below.

The analysis, includes, in some embodiments, an analysis of the digitalimage to be printed, prior to preparing the print job, to assess likelyvisual defects, and the probability of those defects occurring in theprinted article, using physical parameters of parts of the image andtonal contrast between parts of the image.

The printer driver software 11 a then instructs the printer head 10 toprint the image using the selected print mode.

In some embodiments the printer driver can be considered to beintelligent, as it can dynamically and automatically select an optimalprint mode to be used when performing a print job. The optimal printmode can be selected without the user's input, and can save paper, inkand time.

In some embodiments the printer driver software 11 a may be stored inthe memory 12 of the printer 4, and the central processing unit 11causes the printer driver software 11 a to be executed from memory 12.

In other embodiments the printer driver software 11 a may be stored inan off-printer memory and/or executed by an off-printer processor thatis in communication with the printer.

The print mode automatically selected by embodiments of the presentinvention can take into account image quality issues specific to theimage to be printed that a user may not be aware of when selecting aprint mode. For example banding, grain, etc.

FIG. 2 shows a system for printing a document according to an embodimentof the present invention comprising a workstation 2 and a printer 4. Theworkstation 2 includes a personal computer (PC) 3 that is connected tothe printer 4. The workstation 2 includes a user interface including ascreen 5, a keyboard 6 and a mouse 7. The PC 3 has a processor 3 a, amemory 3 b, and I/O software devices 3 c by means of which the processorcommunicates with the screen 5, the keyboard 6 and the mouse 7 and acommunications port 8 by means of which it communicates with a networksuch as the Internet or a local network such as a LAN having peripheraldevices and/or other computers (e.g. PCs), or a WAN, or a MAN, forexample.

The PC 3 also comprises printer driver software 9 that allows the PC 3to communicate with, and instruct, the printer 4. In other embodiments(for example the embodiment shown in FIG. 1) the printer driver softwareis located on the printer 4 itself, and not on the PC 3.

In use, a user uses the workstation 2 to design a document, or the userselects a pre-designed document that they desire to be printed. When theuser selects a print option using the workstation, the printer driversoftware 9 prepares a print command to be sent to the printer toinitiate the print job. The printer driver software 9 disregards a printmode that a user may have selected as part of the print option, or adefault print mode set for the printer, and analyses the document thatis to be printed to determine an optimum print mode for the specificdocument that is to be printed (and sets the print mode to be theauto-selected one).

In some embodiments a user can override a print mode that has beendetermined as optimal for the print operation. The user may be able toselect a print mode that they desire to be used, and force the selectedprint mode to be used irrespective of which print mode is determined asoptimal by the printer driver software.

In some embodiments, the optimum print mode may be determined for eachindividual page of the document, and each page of the document may beprinted with a different print mode if it is determined optimal. Inother embodiments, the optimum print mode may be selected for thedocument as a whole (e.g. multi-page document), and may involveselecting the print mode that is associated with the page that requiresthe highest image quality print mode in order to ensure that every pageis printed with an acceptable image quality.

In some embodiments the printer driver software may be split between thePC 3 and the printer 4. Document analysis processing and/or optimalprint mode selection processing may be split between, or shared across aprocessor on the PC 3 and a processor on the printer 4.

FIG. 3 shows a flow diagram of the steps carried out when printing animage according to an embodiment of the present invention. At step 50, auser selects a print option to print an image. The user may have createdthe image themselves on a PC, or otherwise selected an image to beprinted.

At step 52, the image is notionally divided into a grid of sectors, e.g.square sectors, that cover a large part of the image (and in manyexamples they cover the entire surface of the image). Each sectorcorresponds with a square sector of a known size in the physicalprintout, and the size of the sector is selected to correspond with aminimum area in the physical printout in which banding can be perceivedby the human eye.

In some embodiments the size of the sector may correspond with squareareas in the physical printout that are of the order of 2 cm by 2 cm,1.5 cm by 1.5 cm, 1 cm by 1 cm, 0.5 cm by 0.5 cm, or any other dimensionthat can be set to define what the desired image quality for the printjob should be. Alternatively, a user may define the size of the squaresectors used.

The method then considers a first square sector at step 54. A histogramis computed of the lightness (or darkness) of the pixels, plotting thelightness of a pixel against the number of pixels (i.e. how many pixelsof a particular shade of lightness/darkness there are in the sector inquestion).

The method determines the mean lightness of the pixels within the squaresector, where 100% lightness is white, and 0% lightness is black. Themethod then evaluates whether or not the mean lightness of the pixels isgreater than a threshold level of the maximum lightness at step 56. Inthis example the threshold level is 90% of the maximum lightness.

If the method determines that the mean value of the lightness of thepixels is greater than 90% (the threshold) of the maximum lightness, alow image quality print mode is recorded as being acceptable for thesquare sector at step 60. This is because there is little contrastbetween the printed region and the print medium (assuming that the printmedium is lightly coloured itself) for an area that has been printedwith light inks, and the human eye cannot perceive banding or otherimage quality defects in such lightly coloured regions.

Recording a print mode as being acceptable for a square sector mayinvolve associating in computer memory, for example in a database, anidentifier for the square sector with its associated acceptable printmode. In other embodiments an identifier may not be necessary, and thehighest image quality print mode that has been recorded to date may bestored and be overwritten when a higher image quality print mode isrecorded as being required for a square sector within the document to beprinted (i.e. just keep the highest score to date). This way, thehighest required image quality print mode remains in memory once all ofthe square sectors have been considered.

If the method determines that the mean lightness of the pixels in thesquare sector is less than the threshold (e.g. 90%) of the maximumlightness, the method determines that it is possible that banding mayoccur in the square sector, and the method moves on to evaluate thestandard deviation of the lightness of the pixels in the square sectorat step 58.

The sensitivity of the method to the percentage of lightness is closelylinked to a number of parameters of the writing system of a printer. Forexample, number of inks used to print, type of colorants, media type,etc.

Development engineers can set a preferred threshold percentage oflightness during the development phase of the printer.

If the lightness threshold is too low, the method will not go on toevaluate the standard deviation of the lightness, and will use a lowimage quality (which may be low pass) print mode to print an area fillwhere banding would be visible. This could result in a printout with anunacceptable image quality for the user.

If the lightness threshold is too high, the method will unnecessarilyevaluate the standard deviation of the lightness in an area fill withsuch a light-colour area fill that banding is not perceptible. Thiswould provoke a loss of time in unnecessary algorithm calculations.However, in some embodiments, this loss of time may be preferable to aprintout with unacceptable image quality (in the case of a too lowluminosity threshold). An advantage to considering the mean lightnessbefore the standard deviation is that considering the standard deviationcan be avoided altogether if the mean lightness of the sector is solight that the standard deviation will not effect the image quality forthe particular sector anyway. This can save processing power and time.

If the method determines at step 58 that the standard deviation of thelightness of the pixels is less than a second threshold (e.g. 4%) of themaximum lightness, the method records that a high image quality printmode is required for the sector, at step 62. This is because a sectorwith a low standard deviation has a low spread of pixels with differentbrightnesses. This means that the sector is relatively plain, withoutmuch detail, and banding is more likely to occur in a square with littledetail.

If the method determines at step 58 that the standard deviation of thelightness of the pixels is greater than the second threshold, (e.g. 4%)of the maximum lightness, the method records that a low image qualityprint mode is acceptable for the square sector, at step 64. This isbecause a sector (square in this example) with a high standard deviationcorresponds to a region of the image that has a lot of detail, as thepixels have varying degrees of lightness, and banding is less likely tooccur. Also, the human eye will find it more difficult to perceivebanding in a square with a lot of detail; that is with a high standarddeviation.

Where a low image quality print mode is recorded as being acceptable fora square sector at steps 60 or 64, the quality of perceived visual imagequality will not be substantially affected by whether a high imagequality print mode or a low image quality print mode is used.

After a print mode has been recorded as being acceptable for the sectorat one of steps 60, 62 or 64, the method determines whether or not allsectors of the image (squares in this example) have been considered atstep 68.

If not all of the square sectors of the image have been considered, themethod moves on to consider the next square that has not yet beenconsidered at step 66, and the method returns to step 54 where themethod computes a luminosity histogram for the new square.

If all of the square sectors of the image have been considered by themethod, the method moves on to step 70 where it selects the highestimage quality print mode that has been recorded for any of the squaresthat form part of the image. This may involve searching a database thatassociates each square sector with a required print mode for thatindividual square sector.

The highest image quality print mode is selected to ensure that allsquare sectors of the image are printed without the risk of bandingoccurring in any of the square sectors.

In other embodiments, the highest image quality print mode may not beselected if there is only one sector that requires a higher imagequality print mode than the other sectors. It may be that a user iswilling to risk an image defect in one sector in order to save printingtime and ink for the print job as a whole.

In other embodiments, the highest image quality print mode may not beselected if only 1, 2, 3, 4, 5, 10, or any other number of sectors, or1% 2%, 3%, 4%, 50% or 10%, or any other percentage of the total numberof sectors, require a higher image quality print mode than the othersectors.

Alternatively, a low image quality (which may be low pass) print modemay be selected if a flat area fill is too small for banding to beperceivable in the area.

The image is then printed with the selected print mode at step 72.

In some embodiments, the method will stop analysing sectors and jump tostep 72 to print the document, as soon as the method determines that onesector requires a high (or the highest) image quality print mode. Thisis because a high image quality print mode will be required to print thewhole document irrespective of what print modes are determined asrequired by other sectors. In further embodiments still, the method willjump to step 72 to print the document when a certain percentage of thesectors are determined to require a high image quality print mode. Forexample, the document may be printed when 1%, 2%, 3%, 5%, 10% or anyother percentage of sectors are determined to require a high imagequality print mode. These embodiments have the advantage that processingtime and power are saved by avoiding unnecessary analysis that will notaffect which print mode is determined as optimal.

It will be appreciated that in some embodiments, one or more differentproperties/statistical parameters relating to the pixels within a squaresector may be considered/analysed—the method need not consider both meanlightness and standard deviation of lightness for the pixels: it mightconsider just one of them, or none of them. In some embodiments,properties that can be considered can include one, some, or all, of themean, mode, median, range, standard deviation or any other statisticalattribute of one, some, or all, of lightness, colour intensity, contrastor any other measure of physical characteristics of an image that is tobe printed.

If more than one statistical parameter of the pixels are considered,this may give a more accurate determination of the optimal print modefor a given sector. Alternatively, in some embodiments it may bepreferred to consider fewer statistical parameters of the pixels inorder that computational time and processing power are saved. In someembodiments the consideration of one, two, or three statisticalparameters of the pixels will enable a sufficiently accurate optimalprint mode to be determined.

FIG. 4 shows an example of an image 100 that is to be printed accordingto an embodiment of the present invention. FIG. 4 shows schematicallyhow software is used to notionally divide the entire image 100 intosquare sectors using grid 101. Each square defined by grid 101corresponds to a 1 cm by 1 cm region in the physical printout.

The size of the square sectors are selected so that they correspond witha minimum area in the physical printout in which banding can beperceived by the human eye. That is, if banding occurs in an area thatis smaller than the square sector, it does not matter, as the human eyewould not be able to see it. If the human eye cannot see the banding,then in some embodiments an acceptable image quality has been achieved.

In further embodiments still, the image may be considered as a whole,and the step of notionally dividing the image into sectors is notrequired (the image is equivalent to just one sector).

In some embodiments the software knows the location of blank areas ormargins within the image, and the known areas of the image that consistof a margin or other blank space are not considered by the software, asbanding and other image quality defects will not occur in blank areas ofthe page.

In some embodiments, the software analyzes all of the pixels in thedigital image. If there are blank areas inside the image, or there areblank margins around the image, no more calculations will be done inthese areas because the percentage of the luminosity will be higher thanthe threshold.

The size of the square sector 102 that is determined to provide anacceptable image quality may depend on the intended use of the printoutof the image. For example, the size of the square sectors may be largerfor an image that is to be displayed on a wall and viewed from a longdistance away, than the size of the square sectors for an image that isto be viewed close-up.

In some embodiments there is an additional step in the flow chartillustrated in FIG. 3 between steps 50 and 52. The additional steprequires a user to select the intended purpose of the printed document,and the method automatically selects the size of the sectors that are tobe used in accordance with the intended purpose of the printed document.For example, if a user indicates that the document is to be used forin-depth analysis of a crime scene, the size of the sectors may beselected as a small square with sides of length 0.5 cm. Alternatively,if a user indicates that the document is to be used as a posteradvertisement that will be displayed next to a road, the size of thesectors may be selected as a large square with sides of length 5 cm.

FIG. 5 shows the image 100 of FIG. 4 that is to be printed according toan embodiment of the present invention. Each of the square sectors shownin FIG. 4 are analysed to determine whether or not banding is likely tooccur in that square. FIG. 5 illustrates an analysis of square sector102.

In determining whether or not banding is likely to occur in a square, ahistogram of the lightness/luminosity of the pixels within the square iscomputed. In FIG. 5, histogram 104 shows the lightness of the pixelswithin square 102. The horizontal axis 106 illustrates the luminosity ofthe pixels and has a range of 0 to 255, where 0 indicates completedarkness and 255 indicates white. The vertical axis 108 of the histogram104 indicates a count of the number of pixels that have the lightnessindicated by the horizontal axis.

The histogram 104 is then analysed to determine a mean value for thelightness of the pixels within the square sector 102. If the mean valueof the lightness is greater than 90% (that is in the range of 230 to 255on the scale of luminosity used in the histogram 104), the softwaredetermines that a low image quality print mode will be acceptable forthe identified square sector, and a low image quality print mode isrecorded for the square sector. The software then moves on to considerthe next square sector.

A low image quality print mode is acceptable for a very light squaresector as any banding present in a lightly coloured square will not beperceptible to the human eye. The contrast between the areas printedwith light inks and the colour of the medium itself will not besufficient to cause a reduction in the image quality as perceived by thehuman eye.

In the example shown in FIG. 5, the mean value of the lightness ofsquare sector 102 is 121.71, which is less than 90% of 255 (completelightness), therefore a low image quality print mode cannot be recordedas acceptable for this square at this stage.

It will be appreciated that the threshold of what constitutes a “light”square may be adjusted according to a user's preferences, and may be ofthe order of 70%, 80%, 85%, 95% or 100% complete lightness, any value inbetween, or any other value.

In some embodiments a user can set any threshold values manually using agraphical user interface or other input device, for example a mouse or akeyboard. In other embodiments a computer can set any threshold valuebased on its own image analysis.

If the software determines that the mean value of the lightness of thepixels in a square sector is less than 90% (or any other thresholdvalue), the software can calculate and consider the standard deviationof the lightness of the pixels. The standard deviation provides anindication of the spread of the lightness values for the pixels withinthe square. A high standard deviation indicates pixels with a widevariation of different lightness values, and a low standard deviationindicates pixels with a narrow range of different lightness values.

In other embodiments, the standard deviation can be considered beforethe mean is considered. In some embodiments it does not matter whichorder the statistical parameters are considered in. In furtherembodiments still, the standard deviation and mean (and/or any otherstatistical parameter) can be considered at the same time in parallel.

If the standard deviation of the lightness of the pixels within a squareis less than 4% (that is in the range of 0 to 10 on the scale ofluminosity used in histogram 104), then the software determines thatbanding is likely to occur and a high image quality print mode isrecorded for that square. If the software determines that the standarddeviation of the pixels within the square is greater than 4%, then thesoftware determines that banding is unlikely to occur and a low imagequality print mode is recorded for the square sector.

Square sector 102 in FIG. 5 has a narrow lightness distribution of 3.23as shown in histogram 104, and this causes a high image quality printmode, with typically 8 to 12 passes of the print head (in this example10 passes), to be recorded as necessary for square sector 102 to avoidbanding and other image quality defects.

Banding is more likely to occur where an image has plain, un-patternedregions when compared with images that have a lot of detail. Thecontrast of the lightness of the inks within a square sector reduces thechances of banding occurring (or being noticeable), and this is why ahigh image quality print mode is required for a plain image (that is onethat has a low standard deviation, and hence low contrast in thelightness of the inks), and a low image quality print mode is acceptablefor a patterned image (that is one with a high standard deviation, andhence a high contrast in the lightness of the inks).

It will be appreciated that the threshold for the standard deviation ofwhat constitutes a “plain” square may be adjusted according to a user'spreferences and may be of the order of 1%, 2%, 3%, 4%, 5%, 10%, anyvalue in between, or any other value.

As with the threshold value for the luminosity, the threshold value forthe standard deviation may be optimized for a given printer. Adevelopment engineer may set the threshold value during printermanufacture. This parameter may need to be optimized for a givencombination of print heads, ink colorants, media type and other writingsystem parameters.

In some embodiments the threshold values for mean lightness and standarddeviation may be optimized for the specific printer and/or print mediumbeing used. The optimized values may be factory set: that is, set by anengineer when the printer is manufactured, or when software is installedonto the printer. Different printers may print documents in differentways that cause the printed document to be more or less susceptible toimage quality defects such as banding.

The threshold values (percentage luminosity and/or standard deviationand/or any other characteristic of an image) can be set by performing anumber of tests during the development process of the printer. Fromthese tests it may be relatively easy to find the correct thresholds toselect the most suitable print mode taking into account the media typein which the image is to be printed.

In some embodiments, a printer may self-learn which print modes areacceptable/suitable. For example, a user may indicate to the printerthat a print job was unacceptable, and the printer may repeat the printjob with a higher image quality print mode. In some embodiments, theprinter may keep improving the image quality of the print mode until auser indicates that an acceptable image quality has been achieved. Insome embodiments the printer will store in memory a print mode thatachieved an acceptable image quality, so that a print mode that providesan acceptable image quality is used in future print operations.

FIG. 6 shows the image 100 of FIG. 4, with an alternative square sector202 to be analysed. The mean value of the lightness of the pixels withinsquare sector 202 is shown as 163.16 in the histogram 204. In accordancewith the threshold value of 90% (230 out of 255), this square sector 202is not deemed to be light enough to warrant a low image quality printmode and the standard deviation of the lightness of the pixels is thenconsidered by the software.

The standard deviation of the lightness of the pixels in square sector202 is 3.69, and in accordance with the threshold value of 10 (that is4%) is considered as a narrow lightness distribution. A high imagequality print mode is recorded as being required for this square sector.

FIG. 7 shows the image of FIG. 4 with an alternative square sector 302to be analysed. The square sector 302 defines a region of the imagecontaining a part of a building. The square sectors 102, and 202 shownin FIGS. 5 and 6 respectively contained only regions of sky.

The histogram 304 shown in FIG. 7 indicates a mean value of 170.12 forthe lightness of the pixels within the square sector 302. Again this isbelow the threshold of 230 as defined above, and the standard deviationof the lightness of the pixels is considered.

The standard deviation of the lightness of the pixels within squaresector 302 is 48.65. This is greater than the threshold value of 10 andis considered as a wide lightness distribution. A low image qualityprint mode is recorded as being acceptable for this square sector 302.

FIGS. 5 to 7 show examples of three square sectors 102, 202, 302 thatare analysed to determine the optimal print mode for each specificsquare sector 102, 202, 302. Once all of the square sectors for an imagehave been analysed, the software determines which square sector withinthe entire image requires the highest image quality print mode. Thesoftware then configures the print mode that is to be used to print theentire image as the highest image quality print mode that has beendetermined as necessary for any individual square sector. This ensuresthat the entire image is printed without the risk of banding, or anyother image quality defect, occurring.

The image 100 is then printed with the highest image quality print modethat has been determined as necessary.

In other embodiments, a correlation of which square sector requireswhich print mode is not stored in memory, as in some embodiments thesoftware does not care which square sector requires the highest imagequality print mode. The software only needs information on what thehighest image quality print mode required is, and not which sector itcorresponds to.

In some embodiments it is necessary to divide the document intosectors/zones. If the document was not divided into zones and thestandard deviation of the luminosity histogram was performed on all thepixels of the image, only images with a single flat colour area fillwould be considered susceptible to banding.

In the image shown in FIG. 4, there are regions of plain sky and regionsof grass having a significant amount of detail. If the histogram of thewhole image were considered, the standard deviation of the luminositywould be quite high (there is a wide variety of colours and tones in theimage). This would mean that the image as a whole is not sensitive tobanding and a low image quality (which may have a low number of passes)print mode would be selected. There is, however, a particular area inthe plot (plain sky) that needs to be printed in a high image quality(which may be have a high number of passes) print mode because it isvery sensitive to banding. The detection of this type of area inside animage may only be possible if the image is split into zones, and eachzone is independently analysed.

FIG. 8 shows an alternative image 400 that is to be printed according toan embodiment of the present invention. A first square sector 402 isconsidered, and the luminosity histogram 404 corresponding to squaresector 402 is shown.

In this embodiment the threshold value for the mean lightness is 80%(204 out of 255), whereby a mean lightness value greater than 80% willbe considered as being suitable for a low image quality print mode, anda mean lightness value of less than 80% will be considered as requiringa high image quality print mode. The threshold value for the standarddeviation of the lightness of the pixels is 3% (8 out of 255). That is,a standard deviation of greater than 3% will be considered as suitablefor a low image quality print mode, and a standard deviation of lessthan 3% will require a high image quality print mode.

Histogram 404 corresponds to square sector 402 shown in FIG. 8, andshows that the mean value of the lightness of the pixels within square402 is 78.37, and the standard deviation is 22.88. In accordance withthe above threshold values, the software determines that a low imagequality print mode is acceptable for square sector 402 as there is awide distribution of the lightness values of pixels within the squaresector 402. When printing square sector 402, a faster print mode with 4to 6 passes of the print head will provide the same image quality as aslower print mode with 8 to 12 passes of the print head.

FIG. 9 shows the image 400 of FIG. 8 with a different square sector 502being considered. The mean lightness of the pixels within square sector502 is 174.05, and the standard deviation is 20.58. In accordance withthe above threshold values, the square sector 502 has a wide lightnessdistribution and a low image quality print mode is recorded as beingsuitable for this square sector 502.

Once all of the square sectors have been considered for image 400, therequired print modes that have been recorded for each square areevaluated by the software. The highest image quality print mode that hasbeen recorded is then used for printing the image.

Assuming that all of the square sectors for image 400 have similarlightness distributions to those shown in square sectors 402 and 502, alow image quality print mode is selected as being appropriate forprinting image 400. The entire image can be printed using a fast printmode (4 to 6 passes) while maintaining an acceptable level of imagequality, and avoiding any banding or other image quality defects.

It will be appreciated that the software that performs the aboveanalysis need not necessarily compute a histogram in order to determinethe required statistics. A histogram is shown in the above examples forillustrative purposes. It will be appreciated that computer software canbe adapted to compute the required statistical information from the rawdata associated with the pixels within the square sector by anyappropriate means.

It will be appreciated that the above analysis relates to printing onwhite media where a maximum value of luminosity corresponds with thecolour of the media. It is the contrast between the inks printed ontothe media and the media itself that causes banding, and other imagequality defects, to be perceptible to a user. In other embodiments ahistogram may be computed with a horizontal axis that corresponds to arange of likeness of the colours of the image with the colour of themedium. For example, if printing on blue paper, a square havingpredominantly blue pixels will be considered as not being susceptible toimage quality defects. In such an example, the horizontal axis may beconsidered as a measure of “blue-ness”.

In some embodiments, an image may be divided into sectors of any shapethat covers the entire area of an image without leaving any gaps so thatall pixels of an image are analysed when determining an optimal printmode. For example, the sectors could be rectangles, triangles or anyother shape that tessellates.

In other embodiments, an image may be divided into sectors that arespaced apart, and do not necessarily cover the entire image. Forexample, gaps can be left between adjacent sectors that are small enoughthat banding, or another image quality defect, could not be perceived ifthey were to occur in a gap. Leaving gaps between sectors reduces thenumber of sectors that need to be analysed, which saves processing timeand resource, and can allow a higher throughput of print jobs. In someembodiments the position of the gaps can be selected in regions of theimage that are less important than other regions of the image. Forexample, the focal point of an image may have sectors that abut eachother, and peripheral regions of the image may have sectors that arespaced apart.

In other embodiments, the image may be notionally divided into sectorsthat overlap, in order that some pixels are considered in more than onesector. In this way the method/software can determine the likeliness ofan image quality defect occurring for a pixel (or group of pixels) whenconsidered with neighbouring pixels on different sides. In someembodiments, an image may be notionally divided into overlapping sectorsin important regions of the image, and divided into non-overlappingsectors in less important regions of the image.

It will be appreciated, that in some embodiments an image could benotionally divided into some sectors that overlap, and some that arespaced apart.

Advantages of embodiments of the present invention can include:

-   -   allowing the user to obtain banding-free images without the        requirement of finding the correct print mode;    -   providing faster average printing speeds because “easy-to-print”        images are performed in higher throughput print modes;    -   avoiding the user having to waste time and money searching for        the optimum print mode.

The invention claimed is:
 1. A method of printing a document using oneof a plurality of print modes, the method comprising: notionallydividing a digital image of the document into sectors; for each sector,comparing one or more properties of the luminosity of pixels within eachsector with one or more threshold values; using said comparison of todetermine whether each sector is likely to show an image defect ifprinted at a lower image quality print mode; selecting a print mode suchthat all sectors are determined not to be likely to show an imagedefect; printing the document using the selected print mode; wherein theplurality of print modes cause different numbers of passes of a printerhead of a printer over a print medium on which the document is printedduring a print operation.
 2. A method of printing a document using oneof a plurality of print modes, the method comprising: notionallydividing a digital image of the document into sectors; for each sector,comparing one or more properties of the luminosity of pixels within eachsector with one or more threshold values; using said comparison of todetermine whether each sector is likely to show an image defect ifprinted at a lower image quality print mode; selecting a print mode suchthat all sectors are determined not to be likely to show an imagedefect; and printing the document using the selected print mode; whereina size of the sectors that the document is notionally divided into isselected to correspond with a size in the physical printout in whichbanding cannot be perceived by the human eye; and wherein the pluralityof print modes to perform print operations with an associated pluralityof different image qualities are configured to cause a printer head ofthe printer to perform different numbers of passes over a print mediumon which the document is printed during a print operation.
 3. The methodaccording to claim 2, wherein the sectors are squares with sides with alength of less than, or equal to, 2 cm.
 4. A printer system comprising aprinter, a computer memory having a plurality of print modes storedthereon, and printer driver software; the system being configured suchthat the printer driver software is adapted to compare one or moreproperties of the luminosity of pixels within a document that is to beprinted with one or more threshold values, and to select one of theplurality of print modes that is to be used to print the document basedon the comparison; wherein the plurality of print modes to perform printoperations with an associated plurality of different image qualities areconfigured to cause a printer head of the printer to perform differentnumbers of passes over a print medium on which the document is printedduring a print operation.
 5. A method of selecting a print mode forprinting a document comprising: analysing properties of the luminosityof pixels within the document; comparing the analysis of the propertiesof the luminosity of the pixels with a threshold value; and selectingone of a plurality of print modes for printing the document based on theresult of the comparison; wherein the plurality of print modes toperform print operations with an associated plurality of different imagequalities are configured to cause a printer head of the printer toperform different numbers of passes over a print medium on which thedocument is printed during a print operation.
 6. A method of printing adocument using one of a plurality of print modes, the method comprising:notionally dividing the document into sectors; comparing one or morephysical characteristics of pixels within each sector as they willappear in the printed document, with one or more threshold values;determining one of the plurality of print modes as being the minimumprint quality suitable for printing that sector based on the comparisonof the physical characteristics of pixels for that sector; determiningthe optimal print mode for the document based on the minimum print modesdetermined to be suitable for each sector; and printing the documentusing the determined optimal print mode; wherein the plurality of printmodes cause different numbers of passes of a printer head of a printerover a print medium on which the document is printed during a printoperation.
 7. The method of claim 6, wherein a size of the sectors thatthe document is notionally divided into is selected to correspond with asize in the physical printout in which banding cannot be perceived bythe human eye.
 8. The method of claim 7, wherein the sectors are squareswith sides with a length of less than, or equal to, 2 cm.